My design is a 10x10 drowning chamber with drawbridges on either side, and a 3x3 hole in the floor covered by a retractable bridge. There is a 10x10 4z-level high cistern above it that is constantly pressurized by a 3-pump-wide pump tower connected to the nearby river. There are three levers: arming, drowning, flushing.
The arming lever merely slams the drawbridges shut on both sides, trapping the prey inside.
The drowning lever makes everything turn blue and dead pretty much instantly.
The flushing lever I'm actually rather proud of. Not only does it drain the water almost instantaneously, but all of the corpses and equipment they drop goes down with them, sometimes swirling around the room a couple times on its way. It falls down 10 z-levels (in case of stragglers or water-breathers) onto a grated floor. Below the grates is a large system of channels that lead to edge-of-map drainage fortifications. Pulling the arming lever a second time also unlocks the doors to the collection chamber, and the flood of lizardman leather sock-snatching commences.
It's a dwarfishly large trap, but not too complex. It's probably nothing to brag about, but I'm rather proud of it, as it's only my second attempt ever at actually constructing a trap.
(My first attempt was...bad. Everybody drowned. And then everybody burned. And then everybody was encased in obsidian while the WORLD burned. The survivors, sealed in their burning mountain, dug their own tombs, sealed themselves in, and engraved the walls as the end came.)